Did you deactivate all plugins by
try manually resetting your plugins, using ftp rename plugins folder to plugins_hold and create a blank plugins folder If that resolves the issue, delete blank plugins folder and rename original back to plugins reactivate each one individually until you find the cause.

Did you check error log on server to see what file is causing the problem

Yes, I've tried deactivating all my plugins using both the plugin directory renaming technique as well as editing the wp_options -> active_plugins field in the database but with no luck.

Usually disabling the plugins has worked for me in the past when I've gotten white screens but not this time.

@Paul

I've already tried switching themes by manually editing the database with no result. The public end of the blog switches themes just fine but the Dashboard still won't load regardless of which themes I switch to.

I don't think it's a hosting issue since I have three other WordPress installations on the same host. Their plugin compliment is even roughly the same as the problematic blog's and yet they're working properly.

I've checked for extra blank lines and trailing spaces in wp-config.php (functions.php too) and there are none.

Here's the debug listing:

Notice: Undefined index: HTTPS in /home/myblog/public_html/neb/wp-content/plugins/wp-super-cache/wp-cache-phase1.php on line 526
Notice: Undefined index: HTTP_X_FORWARDED_PROTO in /home/myblog/public_html/neb/wp-content/plugins/wp-super-cache/wp-cache-phase1.php on line 526
Warning: Cannot modify header information - headers already sent by (output started at /home/myblog/public_html/neb/wp-config.php:1) in /home/myblog/public_html/neb/wp-content/plugins/bad-behavior/bad-behavior/screener.inc.php on line 9
Notice: get_settings is deprecated since version 2.1! Use get_option() instead. in /home/myblog/public_html/neb/wp-includes/functions.php on line 3467
Notice: wp_enqueue_script was called incorrectly. Scripts and styles should not be registered or enqueued until the wp_enqueue_scripts, admin_enqueue_scripts, or init hooks. Please see Debugging in WordPress for more information. (This message was added in version 3.3.) in /home/myblog/public_html/neb/wp-includes/functions.php on line 3587
Notice: Undefined property: s2_frontend::$s2_mu in /home/myblog/public_html/neb/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 13
Notice: Undefined variable: post_title in /home/myblog/public_html/neb/wp-content/plugins/wordpress-ping-optimizer/cbnet-ping-optimizer.php on line 78
Notice: Undefined variable: post_type in /home/myblog/public_html/neb/wp-content/plugins/wordpress-ping-optimizer/cbnet-ping-optimizer.php on line 78
Warning: Cannot modify header information - headers already sent by (output started at /home/myblog/public_html/neb/wp-config.php:1) in /home/myblog/public_html/neb/wp-content/plugins/wp-mobile-detector/functions.php on line 690
Notice: Use of undefined constant wprp_init - assumed 'wprp_init' in /home/myblog/public_html/neb/wp-content/plugins/wp-reportpost/main.php on line 15
Warning: Cannot modify header information - headers already sent by (output started at /home/myblog/public_html/neb/wp-config.php:1) in /home/myblog/public_html/neb/wp-content/plugins/wp-super-cache/wp-cache-phase2.php on line 76
Notice: register_sidebar_widget is deprecated since version 2.8! Use wp_register_sidebar_widget() instead. in /home/myblog/public_html/neb/wp-includes/functions.php on line 3467
Notice: register_widget_control is deprecated since version 2.8! Use wp_register_widget_control() instead. in /home/myblog/public_html/neb/wp-includes/functions.php on line 3467
Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /home/myblog/public_html/neb/wp-includes/functions.php on line 3551
Notice: wp_register_style was called incorrectly. Scripts and styles should not be registered or enqueued until the wp_enqueue_scripts, admin_enqueue_scripts, or init hooks. Please see Debugging in WordPress for more information. (This message was added in version 3.3.) in /home/myblog/public_html/neb/wp-includes/functions.php on line 3587
Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/myblog/public_html/neb/wp-config.php:1) in /home/myblog/public_html/neb/wp-content/plugins/si-contact-form/si-contact-form.php on line 2189
Notice: Use of undefined constant wprp_head - assumed 'wprp_head' in /home/myblog/public_html/neb/wp-content/plugins/wp-reportpost/main.php on line 63

I gone through the debug log and disabled every plugin that was reporting an error of some kind.

I also made sure that when disabling SuperCache, I removed its entries from .htaccess.

Yes, I've replaced all the WordPress core files, including the wp-content folder, so the TwentyEleven theme and the Akismet plugin on the server were overwritten with fresh versions from the installation archive.

The blog was recently upgraded from WordPress v3.3.1 to WordPress v3.3.2. After the upgrade, the blog was working fine until last night when it just stopped working. I hadn't made any changes to the blog since it was last fully functional.

Currently, all the plugins are disabled (renamed plugins directory) and I've set the active_plugins field in the database to a:0:{} to be certain.

Here's the debug errors with all plugins disabled, TwentyEleven theme installed, and when I load the home page:

Warning: include_once(/home/myblog/public_html/neb/wp-content/plugins/wp-super-cache/wp-cache-phase1.php) [function.include-once]: failed to open stream: No such file or directory in /home/myblog/public_html/neb/wp-content/advanced-cache.php on line 8
Warning: include_once() [function.include]: Failed opening '/home/myblog/public_html/neb/wp-content/plugins/wp-super-cache/wp-cache-phase1.php' for inclusion (include_path='.:/usr/lib/php') in /home/myblog/public_html/neb/wp-content/advanced-cache.php on line 8

This is the debug errors I get when I try to load to go to /wp-admin/:

Warning: include_once(/home/myblog/public_html/neb/wp-content/plugins/wp-super-cache/wp-cache-phase1.php) [function.include-once]: failed to open stream: No such file or directory in /home/myblog/public_html/neb/wp-content/advanced-cache.php on line 8
Warning: include_once() [function.include]: Failed opening '/home/myblog/public_html/neb/wp-content/plugins/wp-super-cache/wp-cache-phase1.php' for inclusion (include_path='.:/usr/lib/php') in /home/myblog/public_html/neb/wp-content/advanced-cache.php on line 8
Warning: Cannot modify header information - headers already sent by (output started at /home/myblog/public_html/neb/wp-config.php:1) in /home/myblog/public_html/neb/wp-includes/pluggable.php on line 866

@frikafrax sorry for delay as I was busy today I just got back if you look the errors on /wp-admin/

With all plugins deactivated you still have error on wp-super-cache so delete that folder altogether in plugins folder

You have this file in wp-content folder advanced-cache.php delete that one as well and see if that solves the problem.

If you still have problem I would recommend that you backup your database and reinstall from scratch in a new folder and once you have one installed change the database settings in wp-config.php new install to your old database and see if you have problems in this install to rule any problems in database.

If all goes well you rename wp-content folder in new install and copy wp-content folder from your old install to new install and see if this solves your problem.

Deleting Super Cache and advanced-cache.php had no effect but the site's back up and running properly again!

What I did was I re-installed WordPress completely from scratch into an empty directory, confirmed Dashboard was working, imported the original database and copied the plugins directory over.

Before doing anything in WordPress, I edited the database via phpmyadmin to manually de-activate all the plugins by setting wp_options -> active_plugins to "a:0:{}". I also re-saved my permalinks settings.

I copied over the wp-content/uploads directory, then re-enabled plugins incrementally until the site seemed to be displaying and operating normally.

The odd thing was that when I first attempted a fresh re-installation of WordPress using an empty database and installation directory and no plugins copied over, I was surprised to see "headers already sent" error messages and a blank dashboard. But I met with success on the second re-installation attempt. Not sure what happened there...

I'm not sure what the cause of the Dashboard failure was since it wasn't any of the "usual suspects." I.e. theme, plugins, extra blank lines or white space or corrupt core files etc.